Both in Firehole and Madison, Baetis I saw was size 20 to 22.
Then the same Baetis hatch at some selcted spots of Madison both in and out of the Park was a total new experience and a big homework for my future. Friday, Oct 30th, it was cloudy and rainy in the afternoon. I set up nymphs as I have done lots of times before at Upper Haynes. I didn't get any actions. Then I suddenly started to see pods of fish were rising in a slack water including some big backs and heads that would belong to big spawners. I thought "this must be the Baetis hatch that I saw in Craig Mathews and John Juracek's Yellowstone Hatch DVD!!" I was both amazed and excited, then ended up both smiling and frustrated. I understood why local anglers around here can get so excited about this tiny insects' hatch. Just for now, it is good enough for me that I came across this "major league" hatch. I heard that John at BRF caught a 24-inch fall-runner on a tiny dry fly. I believe it.
I did poor jobs. But I did some observation myself regardless of catching or how big they would be. Casting to pods of risers, I figured that they were not taking duns. Instead they were slicing nymphs or emergers just below the surface film. So I tried to swing soft-hackles among the hatch and risers. Nothing!! They even kept rising under my fly line!! This really surprised and frustrated me. Swing method usually works, for example, during caddis hatch and spinner fall. I thought I had to go with dry fly methods. I looked in my vest and I had this "Baetis Foam Nymph". The wing-case is tied with a piece of foam to fish as a dry fly. And most of the times, this pattern is very visible.
When those large run-up trout got close to these tiny flies with a light tippet, I was literally scared, rather than excited or being a cool hunter.
Another pattern that worked was "6X Emerger".
I set the hook with the strength more than needed and broke 6X. I barely managed one of little fish on 6X Emerger. Plus a few more including whitefish.
This whole Baetis experience really tuned up my mind. I'd really better improve my casting and tying these tiny flies. On my very last day at Madison, Nov 3rd, I went to $3 Bridge in the afternoon. Just upstream from the parking lot, I saw the same rising pod in a slick. I raised several trout but they didn't really bite on my offerings. Or was I trying to set the hooks too quickly from excitements? I did hook two of typical Madison trout (14-16 inch ) for a nano-second or so...... It was a sunny warm afternoon, so the hatch and rise seemed to start before 1:00pm when I got there and ended around 2:30pm. I was frustrated but also smiling at the end of the day.